It is generally known in the art that an assembly can be mounted and secured on a support by partially and loosely pre-installing securing bolts in the assembly, and then inserting and locking the heads of the securing bolts into keyhole-shaped openings provided in the support. For example, German Utility Model 297 14 824 discloses an arrangement in which at least one securing bolt is pre-installed in an assembly or component, while a respective receiving opening is provided in the support for receiving the head of each securing bolt. The opening has a keyhole shape, particularly including a first opening portion that is so dimensioned that the head of the securing bolt can pass therethrough, and a smaller elongate opening portion that is wide enough to receive the shaft of the bolt without allowing the head of the bolt to pass therethrough. Thus, the head of the bolt reaches and extends behind the rim of the assembly surrounding the smaller elongate opening portion.
To secure the assembly on the support according to the above mentioned reference, the head of each securing bolt is inserted through the larger first portion of the corresponding opening, and then the assembly is laterally shifted relative to the support so that the shaft of each securing bolt slides into the smaller elongate portion of the opening. Each securing bolt has a respective tool engagement configuration on the head thereof and also on the free shaft end thereof. The tool engagement configuration on one end of the securing bolt allows the securing bolt to be pre-installed in the assembly, and the other tool engagement configuration on the other end of the bolt allows the bolt to be tightened once it has been engaged in the narrower elongate portion of the opening.
While the above described conventional arrangements using a pre-installed securing bolt to be slidingly engaged in a keyhole-shaped opening are convenient for mounting an assembly onto a support, such arrangements all suffer a serious disadvantage, especially when the final tightening of the securing bolt is to be carried out under "blind" conditions, i.e. where the bolt head engaging the keyhole-shaped opening is not visible. Problems arise because the pre-installed securing bolt can be turned tight at any time and at any location, regardless whether or not the bolt is properly positioned relative to the keyhole-shaped opening and the assembly is properly positioned relative to the support.
Under the above-mentioned "blind" conditions, a tool such as a wrench or rachet, and particularly a pneumatic or electric power wrench, can be engaged with the bolt and can tighten the bolt even if the bolt is only partially engaged (or not at all engaged) in the narrower elongate portion of the keyhole-shaped opening. If the securing bolt is tightened in such a position, then the assembly will not be reliably secured to the support, because an inadequate contact surface will be established between the bolt head and the support. Such an improper tightened position of the bolt often cannot be recognized in the above mentioned "blind" conditions, for example when the assembly or installation is taking place in an inaccessible or covered location of a larger piece of equipment, or on a component travelling along a conveyor belt or assembly line. As a result, an improper and failure-prone mounting of the assembly on the support can easily occur and remain undetected during the manufacturing.